Artificial medium of variable dielectric constant



June 24, 1958 2,840,820

ARTIFICIAL MEDIUM OF'VARIABLE DIELECTRIC CONSTANT G. c. 'SOUTHWORTH' 2 Sheets-Sheet 1 Filed April 14, 1954 u. a F E m w FIG. 3/1

FIG. 3B

FIG-.48

IN VENTOR 56. C. SOU THWORTH y W J. M145 ATTORNEY June 24, 1958 G. c. SOUTHWORTH ARTIFICIAL MEDIUM OF'VARIABLE DIELECTRIC CONSTANT 2 Sheets-Shea a 2 Filed April 14, 1954 FIG. I05

FIG. /0A

lNl EN TOR 6;. C. SOUTHWORTH A T TOP/V5 V United States Patent O ARTIFICIAL MEDIUM F VARIABLE DIELECTRIC CONSTANT George C. Southworth, Chatham, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York 7 Application April 14, 1954, Serial No. 423,195

Claims. (Cl. 343-909) This invention relates to artificial media suitable for the propogation of electromagnetic waves and more par- "ice The reflection c'oefiicient at the interface between 1 and n but looking into n is ticularly to media in which the dielectric constant and hence both the intrinsic propagation constant and the intrinsic characteristic impedance may be readily varied.

An object of this invention is to provide dielectric media in which the dielectric constant can be varied by the application of an external force thereby making possible 1) a change in the velocity of propagation inside electric and magnetic components in any wave power.

that may be passing. From one point of view it is permissible to say that re-radiation entails a loss in time and accordingly Wave power transmitted through the medium appears to have traveled more slowly than if no resonators were present. More especially, the velocity has been reduced and accordingly the apparent index of refraction, that is, the ratio of the velocity in free space to the velocity in the medium, has been increased by the presence of the resonators. From an alternate point of view it is permissible to regard the presence of the resonators as having altered the ratio of the electric and magnetic components in wave power that may be passing through the medium. This is equivalent to saying that the presence of resonators has altered the intrinsic characteristic impedance of the medium and hence waves incident upon an interface of such a medium will be reflected accordingly. This invention relates to a method whereby either the velocity of propagation may be altered or the reflection and transmission coefficients at an incident interface may be altered.

The velocity of propagation for free space is given by The transmission coefiicient at the interface between. I

1 and a but looking into n is Numerous artificial dielectric media have already been proposed. One was suggested by Lindemanin Ann. D. Phys., volume 63, No. 7, pages 621 through 644, De'cernber 1, 1921. Other more recent'media weredescribed by W. E. Kock in the Bell System Technical Journal, volume 27, No. 1, pages 58 through 82, Ianuary'1948;-

One form of artificial medium comprises anumberof resonators suspended in space. These resonators may be spheres of appropriate diameter or they may be discs or rectangles or simply wires of the proper length. -In the latter case, the apparent dielectric constant is greatest when linesof electric force-of the incident Wave power a lie parallel to the axis of the wire and least when they are perpendicular thereto. The apparent dielectric constant of the artificial medium depends on thenumber of resonators per unit volume'and also on their proximity to resonance. Specifically, the dielectric constant e, is given by e,- e,,iv-Nafo+f (9) where s is the dielectric constant of the medium in the absence of resonators, N is the number of resonators per unit volume, on is a form factor depending on whether the resonators are spheres, discs, cylinders, or ellipsoids, id is the resonant frequency and f is the operating frequency. Normal operating frequencies are usually below the reof refraction does not vary rapidly with frequency.

It is of interest that in the vicinity of resonance, the

. effective dielectric constant passes rather quickly from very high values to very low values. Theoretically, at

least, it may approach zero or may even assume a negative value. The possibility of using unusual values of dielectric constant in connection with this invention is be rotated to a prescribed angle by any convenient means The invention herewith rests not primarily in the use of suspended resonators, but rather in the use of resonators that are asymmetrical in shape and which may artificial'me'dium may consist of an equilateral parallelo'gram cut from sheet metal and suspended on an axis perpendicular to the principal plane. Alternatively,the resonant element may be an ellipse. The ratio 'of the lengths of the major'and minor axes of the rectangular,

or elliptical figure determines the degree of as'ymmetry.

B y a ymmetry? ism eant a lack of symmetry about an Patented June 24, 19 58 9. axis perpendicular to the plane of the element resulting'in' a configuration having axes of unequal length in the plane of the element. A representative ratio is two, but ,the ratio may beas much as four or more. A bettef understanding of thc invention will best be gained, however, from a consideration of the following description given in connection with the accompanying drawings, in which i Figs. 1A and 1B show planand side views of a res: onator forming a part of the invention;

Figs. 2A and 2B show plan and side views of a housing for the resonator of Fig. 1;

Figs. 3A, 3B and 30 show alternative shapes for the resonator shown in Fig. 1;

Figs. 4A and 4B show two states of the same medium utilizing l l lber of individual resonators; I

Fig 5 shows. an illustrative means for orienting the resonator ofEig. l; V V

Fig. 6 refers to means for, rotating elemental resona tors; I

Fig. ,7 shows. a structure by which the coefficients of reflection and transmission may be varied;

Fig.8 shows schematically how Fig. 7 may be used to switch wave power;

Figs. 9A and 9B show an application. of my invention to prism refractors; and

Figs. 10A; and 103 show an application ofmy invention to wave lenses for varyingthe focal lengthoithe. lens.

Referring now in particular to the drawings. where, by.

way of example, a particular embodiment of the. invention is shown in Figs. 1A and. 113, a metal resonator 11 is mounted on a thin rod or axle 12 which serves as an axis of rotation. For resonator 11, iron is a preferred material but only for the reason that iron elements may easily berotated to any desired angle merely byimpressing a suitable external magnetic field. In order to reduce the. high frequency resistivity of the iron it may be plated with copper or silver. Rod 12 should preferably be a low-loss insulator of-relatively low. dielectric constant. The. metal resonator complete withaxle is then mounted in a thin pill-box compartment '13 providedwith bearings 24 for supporting the axis of the element as illustrated in Figs. 2A and 2B. The two-piece compartment is made of a thin w alled moulding compound of low dielectric loss such as polystyrene and is so .'designed that it can be made very cheaply on a quantity production basis. The finished element, consisting of resonator, axis and support, when assembled, closely resembles an ordinary'pocket compass except in this casethe needle need not necessarily be permanently magnetized. These elements are mounted in close proximity to one anotherin a'thin layer of polyfoarn as suggested by Figs. 4A and 4B. A number of these. layers stacked together constitute the artificial dielectricmedium.

Alternate designs for the resonator include conductors other than iron and also insulators. If insulators are used the dielectric 'constant'should preferablybe very electric field. It is of interest thatthe above principles apply also to the case where conditions inthejartiticial;

medium are reversed, Morep'articularly, my invention applies to an artificial medium inwhich elements of'low dielectric constant are suspended in a medium which would otherwise have a high dielectric constant. Alternate shapes for resonator 11 are shownjn Figs. 3A, 3Band3C. 1

The length of the resonator ll suggested above is not critical. For a: wavelengtht of. ten centimeter s, it may, for

example, he of the order of 2.5 centimeters orapproXl:

mately one inch. For shorter wa lengths, thefiiietal, element should be proportionately smalleri lf the con 4 3 ducting element is coated with a medium of high dielectric constant'the conductor may be made stillsmaller.

In a suggested embodiment of this invention, the elements 11 are suspended in space with their axes of rotation parallel to the direction of wave propagation. For best efiect these elements should be in close proximity to one another. They maybe arranged in plane layers, such as the layer shown in Figs. 4A and 4B. These figures show two conditions (a) and (b) which represent two opposite orientations. of the resonators in a layer relative to the electric vector E in the incident wave power. i

In the first case (a), theelectric vector E lies parallel to the major axis of each aligned resonator and in this condition between the. tips of the, resonators there is a dense concentration of lines of electric force. Though this represents an instantaneous state as the wave passes, it nevertheless correspondsto stored electrostatic. energy, or, more accurately, added electric: displacement density D relative to the applied electric intensity E. Accord: ingly, the effective dielectricconstant.-

is greateron the average than that for the space without resonators.

In the second base (b), the electric vector E lies perpendicular to the major axis of each aligned resonator and in this case the tips across which lines of electric force reside are now more widely separated. Thus, the stored energy between tips is less than before and accordingly the effective dielectric constant is now less. The effective dielectric constant of the medium may therefore be altered merely by changing the orientation of the individual resonators by any convenient means. Itshould be obvious that if the resonators are oriented to an angle intermediate between that shown in condition (a) and that in (b) the effective dielectric constant will be altered accordingly. This constant may be expected to vary more or less in accordance with the cosine of the angle. It will be seen that the two values of the dielectric constant 6 and s depend on the ration of the major and minor axes of the parallelogram which comprises the resonator 11. In the extremecase in which the needle is a slender wire, 6 may be relatively small and the ratio of e to 6 may be correspondingly large.

The discussion above centers about a single layer in the medium. As already indicated, the composite medium may consist of'a'multiplicity of such layers. As should be evident, the over-a1 medium may be shaped to fit a wide range of conditions. For example, it may be made in the shape of a prism for bending a narrow po'rtioriof the wave front. Alternatively, it may be made into a lens for bringing a plane wave to a focus or it may be made cylindrical for use in a wave guide. In the latter case in particular, it becomes possible to taper the dielectric constant on either side so that there will be no abrupt discontinuity. and accordingly no serious reflection from either face. A number of uses for this type of an artifical medium suggest themselves. Others, less obvious, will be described below. i

If a cylinder of this artificial dielectric is placed in a cylindrical wave guide carrying the dominant type of wave andthe active elements or resonators are randomly If the elements are aligned along the electric vector of the advancing wave, the wave .will travel with a velocity of" j a where k isja constant depending-on the" units chosen. Hence, by properly adjusting the length ofthe medium, any desired phase difference may be introduced relative to that that would have prevailed had no medium been introduced. 1 If the elements are aligned in a direction perpendicular to the electric vector, the wave will travel with a velocity Hence for the same length as above there will be a different phase delay. I I

If the elements are aligned 45 degrees relative to the electric vector the two components will travel with different velocities v and v andby a suitable'choiceof lengths, any degree of ellipticity in polarization may be achieved including linear and circular polarization. It is particularly significant thathavingestablished a, given state of polarization, say circular, it is possible .to rotate the individual resonators to produce other degrees of ellipticity including horizontally plane-polarized waves and vertically plane-polarized waves. i

If instead of a circular guidearectangular guide is used and the latter is'proportioned so as to propagate but one component wave,then the .tran'smitted'wave will always be plane-polarized with its electric vector parallel to the short side of the guide. The velocity of this. single wave will be varied as the elemental resonators are rotated. Also the longitudinal impedance of the guide may be varied and substantial reflection effects may be produced. I

Fig. 5 shows a convenient means for orienting resonators 11 in-the event these resonators are madefof magnetic material. The means illustrated here consists of two pairs of electromagnets N -S and Nye-S2 aligned with respect to each other so that two steady-state orthogonal magnetic fields can be generate'din-the' region surrounded by the magnets. By varying the strength of these fields any desired orientation of the magnetic re'sonators can be obtained. It should be understood, however, that in the event resonators lljareinon-magnetic, other suitablerneans' for rotating them shall. be; used in place of this magnetic means;

Figure 6 shows a waveguide containinga matched section of artificialdielectric of the kind-already described. The medium is now surrounded by two- -0rth'ogonally placed pairs of magnets 15, 15 and 16, 16 as before but this time they are energized by a two-phase alternating magnetic field in such away that the'ass'emblage of ele-' mental resonators rotates in space. The'rate of rotation.

is numerically equal to-the frequency of the two-phase energizing current. Alternatively, three-phase alternating fields may be used in accordancewith standard engineering practiceL Rotation of the resonators may also be accomplished by transmitting" through the'medium' circumedium described above, one is speedcd up in its rotation. while the other is retarded. Uponrecombination after] passing through a given length of such a rotating medium, 7

the resultant appears to have been rotated in space. This method of rotating the planeof polarization is somewhat analogous to the mechanism believed'to be operative in the so-calledferrites. Thus the rotation effects observed are"nonreciprocal in'their behavior and like the ferrites they' make possible alarge variety of useful circuit 7 eletnents.-

- Thus far 7 the moving electromagnetic waves the much slower moving waves of sound the planeof polarization may be rotated even with the above-described apparatus in its present state of development; Ordinarily, sound waves in fluids have no transverse components either: of pressure or particle-velocity but when confined to, pipes there may be" set up, as the resultants of multireflections of zig-za'g'corripression waves, very substantial transverse e'omp'onjents.

It is these waves that are utilized in the method herede scribed; It is of interest that we may transmit through the'medium circularly polarized electromagnetic waves and use these to rotate the resonators thereby rotating the plane of polarization of anytransverse sound waves that may be present. An-alternate method of rotating'the planeof polarization of'a-transverse sound wave was described by W. E. Kock, application Serial No. 323,175,

filed November 29, 1952.

"The foregoing disclosure illustrates how variations in the effective dielectric constant of the mediurn'maybe .varied tomodify the velocity of propagation; In this case it was recognized'that reflections may take place at i the incident interface but suitable tapering was'invoked as a means of keeping this to a minimum. Techniques for accomplishing this result are well known and need not be reviewed at this time. ,We now consider cases where we make a virtue of reflection losses. More particularly we use the change in eflectivedielectric constant as described above as a means 'of varying the intri nsic impedance of the medium and hence thereflection coeflicient at its interfaces.

I show as Fig. 7 a section of waveguide 17 containing a plug 18 of artificial dielectric'of the kind described above. It is assumed that the reflecting interfaces corre i.

larly polarized electromagnetic waves, the wave itself causing the rotation. Such a method of rotation lends itself to eifecting the transmission of waves other than high frequency electromagnetic waves, thus effects on the -.transmission" o:f' 'soi1nd fwaves' maybe accomplished; It Qissignifieant thatthe"rotatinggconfigurationiof reso'na: torsemaybe usediogotate the planeofpolarization of" any plane-polarizedj waves that"may be passing through the medium. Y I x It was shown years ago that' a. plane-polarized wave isfequivalent to two circularly polarized waves,'one 1 rotating clockwise and the other counter-clockwise; When these two components pass through. the rotating i the} length of the sponding to the two .ends of the plug areeach plane .sur faces perpendicular'to'the axisof the guide. It is further assumed that the unit resonators maybe-oriented as desiredby a mechanism like that already shown in Fig. 5.

- If thef'resonators are oriented to give an effective dielectric constant 6 corresponding to thelower of the two limits available, it may be expectedto produce' anappreciable reflection coefficient at the two.interfaces. If this is undesirable, it may be cancelled by introdu cing near each end" an jris diaphragm 19, 19 01: suitable diameter and spacing. Under'this condition the coefficient of transmission willjbe substantially unity. 1

'If next'we reorient the elemejntary resonators to produce the higher dielectric constant e then. a very substan- -tial reflectiorr'coeflicient Will appearQand the' coeflicient of transmission iwill be reduced accordingly g Since re flec tionsgtake' place'frorn b'oth ends of the plug we, should preferablyadjust'the length of the plug so that the'two, reflected componentsappear on the incident side in.the I same phase. This favorable condition willobt'ain when plug is' e'qual to an odd 'number of quarter'waves.

above methods have been described as though they were applicable only to transverse electromaterial.

assasao 11: will be v den tha cir ui ccmrsaeat likethatii s de c ibed are in fi t Wa w tche a tltl m ib used. in a v r y. qflways- One imP s ai sit l l s trated in Fig. 8 where -w ave power'from "la-guide A is transferred ,inany desired prgportiontto either guide e ri ed arepl s in ll??? an Quil @9 Ft 0i circuits so wired that one unit opcnslasg c other closes.

Eigures9A-and 9B are illustratiye of application of this invention to a retracting" prism 21, whereby the rera fir lefies @tth Qr saima be ar n Fig! 9 described. Fig. 9B shows the elements rotated to the"? position where the dielectric constant of the prism, and hence the index of refraction; is at a. minimum.

Thejsame principles'of operation may' be utilized in the case of a lens or focusing element 22 to vary the focal engt t e eo Q-Fisl A S Q 'Ih s ms r l t ashort focal lengthfposition for a converging lens, that is,

the re ract v ind ss a s h focal en t m y b inerea sed'to a maximunras shown in Fig. B rotation of the elements 11, in .the same manner as discussed in connection WithiEigs QA-and 93. An interesting fact to be noted with respect to Figs; 10A and 10B is that a continuously variable focal length; lens maybe had by application of a rotating magnetic field. t

,The above description is given in illustration and not in limitation of the inventirm. It should be understood in particular that the shapes and spacings of the individual resonators are not limited to those shown but may include others as well. It shouldbe understood also thatwhile described in connection with transverse electromagnetic waves, this rnethodapplies quitegenerallylto all types of wave motion including particularly sound wavespropagated through fluids enclosed .in pipes havingiigid walls.

Various changes and modifications in the invention and in the ways of utilizingit will occur to those skilled in the art and these changes or modifications may be made without departing. from the spiritior i scope of the inventi-on asset forth in the appended claims.

What pla eau 1 a 1. An electromagnetic wave transmission medium com;

prising one or more layers of composite materialfand havingadielectric constant, each of said layers including a number of individual rotatableelement's lyingsubstantially in a plane and spaced apart. therein, each element having an axis of rotation perpendicular to. saidfp lane; each elementformed by a piece of material ha ving an asymmetrically. hape urface perpendicular to the di rection of wave propagation 'thro11gh":said medium} and means for orienting each of said elements' relative f to the electric field of a wave propagatinglthrbugh' said medium :wherebyit'he effective dielectric constant of said medium can be varied in accordance Witha signal. i i

2. electromagnetic Wave transmission mediurrn as claimedin claim 1 in which atrle'ast some of said Qelc ments'arethin diam-ond'shapedpiec'es of magnetic meal. 3. An electromagnetic wave transmission medium] as claimed in claim 11in which' atllea'st some of said elements I arethin pieces tjf {high ljiiielectric 4.QAn electromagnetic wave transmission medium "as claimedin claim 1 in which the 'individualieletn' n s iii each layer are uniformly spaced apart. i '5. Au' electromagnetic wave transmission 'medium as claimed in claim l in which the ratio of. length to width of atleast'some 'of said elements is greater than 'one'. 6. An electromagnetic wave transmission-mediumas claimed in claim 1; in which -at least some of. said elements alie pr c ly hapgd m .i t

7QA'n eIec'troma'giieti'c wave transmission medium as claimed in claim 1 in which each of said layers of composite material is spaced from an adjacent layer of said material in the direction of wave propagation.

8. An electromagnetic wave transmission medium comprising one: or fn'iore layers "of composite material and having a dielectric"constant,--;achof said layers including' a number bf rotatable elements lying substantially in a plane "and spaced apart thereinf'each element having an-axis of rptationperpendicularto said plane; each eleriient formediby apiece of materialhaving 'anf'asymmetrically"shaped surface perpendicular to the direction of wave propagation through said medium, and means comprising a magnetic field for orienting each of said elements relative to the electric field of a wave propagating through said medium whereby theeflfective dielectric constant of said medium can be varied in accordance with a signal;

"'9. An electromagnetic wave transmissionrmedium as claimed in claims in which the magnetic field is a contiriuously'rotating field. a 1 I V 10. An electromagnetic wave transmission medium for altering-the direction of propagation of'wave energy incident thereu'pon comprising a prism shaped member of composite material ahaving a dielectric constant, said material compri ing-aplurality of asymmetrically shaped rotatable elements having axesof rotation Parallel to the direction 'offlgv'ave zprop'agation through; said prism, and means for variably orientingsaidelements relative tothe wave energy wherebyzthe index of refraction of said prism is varied.- I f 11. An electromagneticwave transmissign mcdium for focusing wave energy incident thereupon comprising a convex lens shapcd'mernber of composite material having adielectric constant, said material comprisin a plurality of asymmetrically shaped rotatable elements having a res ofrrotatipn'parallelto the direction of wave propa gauon 'therethrou'gh, and means for variably orienting said elements relativeto the wave energy whereby the focallength of the lens shapedme mbcr is varied. i

12; An electromagnetic wave transmission medium according'to'c1aim -lf1iwherein the me ans *for variably orienting said elements comprises a continuously rotating magnetic field whereby-the focal lengthofthe lens shaped member is continuously'varied.

13.'Av.-compos,ite dieleetnic mernber including a plurality of asymmetrically s apedresonatorsof the order of one-quarter wavelength long spaced apart from one another andr oriented in such a manner as to produce in said e n fiec v l tr sqns a greater than one, and means for yarying the orientation of said resd nators 'whereby the dielectricconstant of said member may be varied, Y i

14. VA 'wave propagation m dium comprlsing one or morevlayersgof composite material and having a propagation constantgyeach of said layers including a number of individual rotatable 'l elements lying substantially in a plane and spaced apart therein, each element having anv axis ofrotation perpendicular to said plane, each element formed by a piece tof rnaterial having an asymmetrically shaped. surfacc pe'rpendicular to the direction of wave propagation through said medium, and l i orienting: each of said elements whereby h @fiectlve propagation constant of said medium canlbe' varied.

.15.'-A:' wave propag tio wn fid ll? ,Q I P El L P 9 or more layers oficomposite m teriaI andhaVmg a propa- 1- in ia number plane and spaced apart therein, each; element liaving an axis" of rotation perpendicular to said plane, each element f rmed; by a: .pirj;ce of;m ii1 having an nsymmetrically shapednsiirface. p erpendicuiar to the directiqn ofwave propagation through said mediurn, and means cnrnpnsln btzhitially in i a be varied.

References Cited in the file of this patent UNITED STATES PATENTS Southworth Sept. 13, 1938 10 Purcell Aug. 19, 1952 Varela July 7,' 1953 Kock Sept. 15, 1953 Cohn Feb. 2, 1954 Cutler Feb. 16, 1954 Zaleski July 27, 1954 

